1. Field of the Invention
The present invention relates to a method for calibration of an optically variable filter array apparatus suitable for use in selection of WDM signals.
2. Discussion of the Related Art
At present ROADM (Reconfigurable Optical Add Drop-multiplexing) nodes for WDM communication systems are required to offer a sophisticated colorless Add-Drop function, or equivalently the capability of effecting separation and addition of optical signals with given frequencies. As a method for the attainment of the colorless function, there is known a configuration using a tunable filter array (TFA) for selecting a plurality of input signals from among input WDM signals. In Japanese Unexamined Patent Publication JP-A 11-23891 (1999) and Japanese Unexamined Patent Publication JP-A 5-196910 (1993) as well, there is proposed, as a tunable filter array structure of conventional design, an optically variable filter that utilizes variation in cavity length resulting from an electrical-field effect in a liquid crystal element. Moreover, in Japanese Unexamined Patent Publication JP-A 2000-28931 and U.S. Pat. No. 6,449,410 as well, there is proposed a wavelength variable filter for changing selected wavelengths with variation in cavity length resulting from mechanical alteration in MEMS.
Since a WDM modulated signal exhibits a spread of spectral components on a frequency axis, as a filter configuration, a flat-top spectral waveform pattern is desirable that is characterized by inclusion of signal spectral components and low crosstalk between adjacent channels. However, the filter configuration of a Fabry-Perot interferometer is of Lorentzian type, which is unsuitable for a filter for selecting specific wavelengths of WDM signals because of too narrow a peak range of a selected wavelength. Furthermore, in the filter disclosed in US 2009/0263141A1, power consumption is increased for array configuration due to the exploitation of TO effects. In addition, in the filter presented in Conference paper TuN2 of OFC 2003 by J. Berger, F. Ilkov, D. King, A. Tselikov, and D. Anthon, the filter configuration is of Gaussian-waveform pattern and is therefore, just like others of the conventional art, unsuitable for a filter for WDM signals.